A new study suggests that Saturn’s moon Titan has a thick, rigid crust of ice, a finding that confounds scientists whose explanations for the moon’s dynamic surface and atmosphere rely on a flexible crust.

“Already things on Titan were hard to explain. This makes it even worse,” says study coauthor Doug Hemingway, a planetary geophysicist at the University of California, Santa Cruz. “It deepens the mystery of a very strange body.”

Titan is Saturn’s largest moon and sports a thick atmosphere. NASA’s Cassini probe, which has orbited Saturn since 2004, has peered through Titan’s orange haze to discover methane and other hydrocarbons raining down to fill vast lakes as well as freezing to form soaring dunes. Cassini also has collected evidence for an ocean of liquid water, separated from the surface by an icy crust.

Hemingway and his colleagues performed what they thought would be a routine analysis of two new datasets from Cassini: one tracking Titan’s gravity and the other its topography at different points on the surface. They expected that Titan’s hilly regions, where the crust is thickest, would have slightly higher gravity than low-elevation areas because of their extra mass. But Hemingway was surprised to find the opposite connection – the highest regions have the lowest gravity. “We thought we had a sign backwards in the math,” he says.

After ensuring the analysis was correct, Hemingway’s team set out to explain the strange elevation-gravity relationship. Their attention quickly turned to the boundary of icy crust that sits on underlying ocean. Because ice is less dense than liquid water, the thickest regions of crust should be more buoyant, causing them to rise and create mountains on the surface. But that can only happen if the crust is either thin or flexible enough to allow that motion.

When the researchers plugged the topography and gravity readings into equations, they found that Titan’s ice shell is likely too thick and rigid to allow localized regions of thick crust to rise. Instead, those thick chunks remain submerged. Due to their low density, they have less mass than the water they displace, thus reducing the gravity.

In addition, Hemingway’s team predicts that Titan’s wind and rain cause erosion that transports mass from mountain peaks to troughs below, further enhancing the seemingly backward elevation-gravity connection. The findings appear in the Aug. 29 Nature.

Understanding the thickness and rigidity of Titan’s ice shell is crucial to figuring out what fuels the moon’s methane rain, lakes and dunes. Titan’s atmosphere contains so much methane that if it all rained down, it would form a global layer 10 meters thick. Yet scientists have no idea where that methane comes from. Until the new study, the most logical origin for methane seemed to be deep beneath the surface, says Cornell University planetary scientist Jonathan Lunine. But it would be very difficult for methane to vent through a thick, stiff crust into the atmosphere.

Lunine says he hopes that scientists can solve this mystery before 2017, when Cassini will plummet into Saturn to end its mission.